Pollution Variability at an Urban Junction Tom Bentham, Roy Colvile, Chris Pain & Alan Robins Levels of urban air pollution are usually quantified in terms of concentrations averaged over a long period (an hour or more). Dispersion models and routine air pollution monitoring rarely resolve scales smaller than 100m. Pollution concentrations can, however, vary sharply over much shorter spatial and temporal scales than these.The aim of the work presented here is to quantify levels of variability in urban air pollution concentrations. This is the first step towards answering the question: is the long-term average concentration a relevant and sufficient measure of urban air quality? Dispersion at the intersection of two symmetrical street canyons was studied in the wind tunnel, with one canyon parallel to the mean flow and the other perpendicular. Two sources were investigated: a point source positioned in the upwind street, and a line source along the length of the perpendicular street. Numerical modelling of dispersion at the intersection, using adaptive-mesh large-eddy simulation, is under development. Fine-scale spatial concentration variability at the junction is evident with both the line and point sources – see Figures 1(a) and 2(a). Rounding a corner at the junction, the mean concentration changes by almost an order of magnitude, from 4.0, in both cases. The ratio of 90 th percentile of concentration to mean, shown in Figures 1(b) and 2(b), is a measure of temporal variability. This ratio is highest in the street which does not contain the source, analogous to a side-street opening onto a busy road. In both cases, the ratio exceeds ten in places, indicating that concentration frequently exceeds the mean by an order of magnitude. These results are for an idealised situation with a single well-defined source, but they are applicable to real urban situations. The line source simulates a busy road, while the point source represents a particularly polluting vehicle or a single harmful release. Intense variations in pollutant concentration have been found over small spatial and temporal scales, of the order of a few metres or seconds, implying that some people will be exposed to concentrations much higher than the mean. Future work, including DAPPLE, will indicate how best to incorporate variability into air quality standards. Figure 2: Tracer concentrations due to a point source in the ‘left-hand’ street (a) mean concentration (b) r atio of 90 th percentile of concentration to mean (expressed as log 10 ) Figure 1: Tracer concentrations due to a line source in the ‘vertical’ street (a) mean concentration (b) r atio of 90 th percentile of concentration to mean (expressed as log 10 ) In both figures the free-stream flow is from left to right, the cross-sections are at a quarter of the building height and concentrations are expressed non-dimensionally. To find out more, contact Tom: Department of Environmental Science and Technology, Imperial College, London SW7 2BP tel: